1. Field of the Invention
The invention relates generally to fixed cutter drill bits used to drill boreholes in subterranean formations. More specifically, the invention relates to methods for modeling the drilling performance of a fixed cutter bit drilling through an earth formation, methods for designing fixed cutter drill bits, methods for improving and/or optimizing the drilling performance of a fixed cutter drill bit, and to drill bits formed using such methods.
2. Background Art
Fixed cutter bits, such as polycrystalline diamond compact (PDC) drill bits, are commonly used in the oil and gas industry to drill well bores. One example of a conventional drilling system for drilling boreholes in subsurface earth formations 100 is shown in FIG. 1. This drilling system includes a drilling rig 101 used to turn a drill string 103 which extends downward into a well bore 105. Connected to the end of the drill string 103 is a fixed cutter drill bit 109.
As shown in FIG. 2, a fixed cutter drill bit 111 typically includes a bit body 113 having an externally threaded connection at one end 115, and a plurality of blades 117 extending from the other end of bit body 113 and forming the cutting surface of the bit 113. A plurality of cutters 119 are attached to each of the blades 117 and extend from the blades to cut through earth formations when the bit 111 is rotated during drilling. The cutters 119 deform the earth formation by scraping and shearing. The cutters 119 may be tungsten carbide inserts, polycrystalline diamond compacts, milled steel teeth, or any other cutting elements of materials hard and strong enough to deform or cut through the formation. Hardfacing (not shown) may also be applied to the cutters 119 and other portions of the bit 111 to reduce wear on the bit 111 and to increase the life of the bit 111.
Significant expense is involved in the design and manufacture of drill bits and in the drilling of well bores. Having accurate models for predicting and analyzing drilling characteristics of bits can greatly reduce the cost associated with manufacturing drill bits and designing drilling operations because these models can be used to more accurately predict the performance of bits prior to their manufacture and/or use for a particular drilling application. For these reasons, models have been developed and employed for the analysis and design of fixed cutter drill bits.
Two of the most widely used methods for modeling the performance of fixed cutter bits or designing fixed cutter drill bits are disclosed in Sandia Report No. SAN86-1745 by David A. Glowka, printed September 1987 and titled “Development of a Method for Predicting the Performance and Wear of PDC drill Bits” and U.S. Pat. No. 4,815,342 to Bret et al. and titled “Method for Modeling and Building Drill Bits,” and U.S. Pat. Nos. 5,010789, 5,042,596, and 5,131,478, which are all incorporated herein by reference. While these models have been useful in that they provide a means for analyzing the forces acting on the bit, their accuracy as a reflection of drilling might be improved because these models rely on generalized theoretical approximations (typically some equations) of cutter and formation interaction. A good representation of the actual interactions between a particular drill bit and the particular formation to be drilled is useful for accurate modeling. The accuracy and applicability of assumptions made for all drill bits, all cutters, and all earth formations can affect the accuracy of the prediction of the response of an actual drill bit drilling in an earth formation.
In one popular model for drill bit design, it is assumed that the centerline of the drill bit remains aligned with the centerline of the bore hole in which the drill bit is drilling. This type of centerline constrained model might be referred to as a “static model,” even though the model calculates incremental dynamic rotation. The term static as applied to this type of modeling means not varying centerline alignment. In such prior modeling and fixed cutter drill bit design, there was little focus on the use of side rake angles of the cutters to improve performance of the drill bit. The focus was on the rate of penetration obtained and thus the cutter layout, position, and back rake angle were the parameters of predominate interest to improve cutting performance.
Fixed cutter drill bits are desired that have side rake angles and distributions of side rake angles that provide improved performance and stability. A method is desired for modeling the overall cutting action and drilling performance of a fixed cutter bit that takes into consideration and uses side rake angles and/or side rake angle distribution of cutters along blades of fixed cutter drill bits to improve and to optimize drill bit performance not only for rate of penetration but also for dynamic stability so that a desirable rate of penetration can be maintained during drilling.